This invention relates generally to filters used for ultrafiltration of slurry mixtures which also require separation of oil, other liquids, or gas from the slurry to be filtered. This invention more specifically involves a high-speed rotary disk filter made from sintered particles, where the filter disks have expungement passageways.
Ultrafiltration, sometimes referred to as hyperfiltration, involves the removal from a fluid of particles having a size on the order of 10 microns or less. Conventional filters utilizing filter elements such as wire mesh are not suitable for ultrafiltration. Satisfactory wire meshes are generally not available at 10 microns and below because of the small size and the difficulty in achieving uniform spacing.
Applicant's own application Ser. No. 559,744, filed Dec. 9, 1983, discloses a nonclogging high speed rotary disk filter for ultrafiltration. That invention avoids the problem of clogging which has plagued conventional filters used in ultrafiltration applications. Several porous disks were provided on a hollow rotatable shaft. Filtrate would be introduced into a tank containing the disks rotating at high speed. Filtered fluid flowed through the disks, into the hollow shaft, and out a conduit for removal, while the fluid to be filtered continued to be circulated in the tank.
Applicant's rotary disk filter is rotated at high speeds sufficient to establish a boundary layer or barrier layer in the fluid itself, which inhibits the migration of small particles from the slurry into the filter element. When the shaft is rotated at an appropriate speed, the barrier layer created adjacent to the exterior surface of the filter disks effectively shields the surface from the particles. The particles are inhibited from crossing the barrier layer which was established in the fluid near the surface of the filter disk. In applicant's high-speed rotary disk filter, much of the filtering action occurs at the barrier layer in the fluid, and not in the filter element itself. This avoids the problem of clogging. The problem of clogging has plagued the prior art where small undesired particles from the fluid typically collect to form a cake on conventional filters. The cake of particles is usually sufficient to severely reduce or completely block the flow of fluid through the filter element. This typically resulted in expensive down time to clean the filter, as well as expensive maintenance and cleaning operations.
Significantly, by creating a barrier layer, Applicant's high speed rotary disk filter avoids the requirement of frequently interrupting filtering by backpulsing the filtrate. A rotary disk filter has been proposed by Breton et al. in U.S. Pat. No. 3,997,447. But Breton et al. provided that at predetermined intervals the fluid flow through the filter must be reversed or backpulsed. Breton et al. failed to discover the dramatic advantages achievable when a barrier layer is created. Breton et al. used disks that were too small (2 inches in diameter) and which were rotated too slow (1325 rpm in liquid) to create a barrier layer. In fact, the frequent backpulsing taught by Breton et al. would create turbulence that would destroy a barrier layer. Breton et al. creates turbulence; Applicant's high speed rotary disk filter avoids it.
The design of Applicant's rotary disk filter is such that for a given flow rate, the size, weight, and space requirements are considerably reduced when compared to conventional filters. The economic benefits of Applicant's rotary disk filter are achieved by reducing pumping requirements, eliminating cartridge or filter element replacement, minimizing clogging, expensive down time and the need for backpulsing to clean the filter element, and improving the quality of filtration.
Applicant's rotary disk filter was originally intended to be operated in a horizontal position; that is, the axis upon which the rotary disk filter was rotated would generally lie in a horizontal plane.
Applicant discovered that by providing relief holes through the disk, and by operating the rotary disk filter in a vertical position, oil, some other liquids, and gaseous matter could be removed from a slurry during filtration. To Applicant, this discovery was unexpected and surprising.
In the past, oil and water suspensions have been separated by centrifuges. Centrifuges have been inconsistent in separating stable emulsions. If the concentration of oil and water varies in the input feed stream, a centrifuge would require continual readjustment for efficient separation. A centrifuge normally depends upon there being a significant difference between the densities of two liquids being separated. A centrifuge typically will not work well if the densities of the two liquids is nearly equal.
A stationary cross flow filter, such as provided in Applicant's prior application Ser. No. 365,836, may be used to separate water from oil mixtures by first wetting the porous filter with water. The oil and water mixture must be passed at a high velocity, under pressure, over the porous filter membrane. Generally low pressure must be used, because excessive pressure will cause both oil and water to pass through the filter. This low pressure results in low flow rates of filtered water. Much energy must be expended in a typical cross-flow filter to pump an adequate volume of liquid necessary to sustain the high velocity of flow across the filter membrane which is essential to filtration. Only a small portion of the total flow actually passes through the filter. Such a process is inefficient.
A critical range of several parameters should be observed in order to maximize the advantages of the present invention.
The present invention rotates porous disks through the filtrate. The surface velocity of the rotating disks must exceed a certain lower limit in order to achieve proper operation. A series of holes or slots are located along the vertical shaft that supports the disks. The disks are spaced an appropriate distance apart, preferably 5/8 inch. The disks are rotated at speeds in the range of 500 to 1000 rpm, based on a 12 inch diameter disk, to achieve a minimum disk surface velocity of at least 15 feet per second over a major portion of the disk surface. This is necessary to establish a barrier layer in the fluid to be filtered.
The oil that is more intimately bound in the water phase enters into the area between the disks. Centrifugal forces, and shear forces at the barrier layer, assist the coalescence of small droplets by reducing surface tension. These forces act to cause the lighter oil droplets to migrate toward the shaft. More dense fluids migrate toward the walls of the tank. The oil droplets, due to their lower specific gravity and buoyancy, rise or migrate along the shaft through the relief holes in the disks. The oil is discharged from the top of the tank. The relief holes are as close as possible to the shaft, where the radial velocity of rotation is smallest.
A pressure differential is applied between the hollow shaft and the tank. This pressure differential encourages water to diffuse across the barrier layer and into the filter disks. But the pressure should remain within an appropriate range. If the pressure is too great, oil may be forced into the filter.
The speed of rotation of the disks and the size of the disks are interrelated factors that are critical. If the disks are too small, they will not achieve enough surface velocity to establish a barrier layer or the shear forces necessary for oil and water separation. This may be partially compensated by higher rotational speeds to achieve greater surface velocity at a given diameter, but there are limits beyond which this cannot be done.
Thus, the invention requires the use of oil relief holes in porous disks mounted upon a vertical shaft, under critical ranges of pressure, rotational speed, disk size, and disk spacing.
A rotary disk filter constructed in accordance with the present invention allows ultrafiltration to be performed upon a slurry, while at the same time removing oil, some other liquids, and gaseous matter from the slurry. A rotary disk filter constructed in accordance with the present invention minimizes clogging, minimizes expensive down time, substantially eliminates cartridge or filter element replacement, reduces pumping requirements, minimizes back pressure, and substantially minimizes the need for backpulsing to clean the filter element. Such a filter can efficiently handle variable concentrations of oil and water, for example, without adjustment. The filter will separate two liquids which have densities that are close to each other. The filter will handle small concentrations of fluid to be filtered. Other features and advantages of the present invention will become apparent from the following detailed description of a presently preferred embodiment.